In Halo CE and Halo 2, the main objective was to stop the rings from being lit and to destroy them. Why in Halo 3 did that change at the end and Chief told 343 Guilty Spark he was going to light the newly made one?
They had to light the new Installation 04 to destroy the local infestation, and the Ark. The Portal was also still open, so they could light the ring and than quickly get out of there.
Since the Ark is so far away from the Milky Way, the Chief new that firing just 04b would be fine. It would destroy the local infestation. What he didn’t know was that it would destroy the ring and the Ark. That’s why Spark wigged out.